The story of Jihadi John underlines the grave dilemmas faced by MI5

Mohammed Emwazi was born in 1988, the son of a Kuwaiti policeman who moved to
Britain in 1993. He and his family lived in Maida Vale, where he was an
undistinguished schoolboy with a passion for Manchester United, and later an
IT student. He attracted the attention of the security services when he
began to associate with Muslim radicals, and subsequently spent hours in
airport interrogations during journeys to Europe, Tanzania and Kuwait.

In 2012, he managed to travel unnoticed to Syria, where, in 2014, he achieved
unspeakable notoriety for an Isis video in which he cut off the head of
American journalist James Foley with a knife. He subsequently beheaded four
more western captives, becoming known as “Jihadi John”, one of the foremost
targets for western retribution.

In 2015, MI6 achieved a breakthrough by discovering that Emwazi had married
the mother of his young